What would cause the grid heater (wait to start light) to cycle in 80+* heat, while driving down the road at opperating temp, this just started to happen on my dad's 1991 last week while they were on vacation (happened around 5 times). Most of the time, even in the winter it does not come on, any help would be great. Thanks

 

Is it effecing the way the truck runs at all? That's weird, mine doesn't ever come on either unless it's winter. Sounds like a short or somethin done gone looney. Definitely shouldn't be comin on while your driving

 

i'd guess that one or both of the relays are shorted causing the trigger circuit to be by-passed. they have a fairly common way of doing that. alot of people have woken up to trucks with dead batteries because of this.